The present invention generally relates to a dental post and core system for endodontically-treated teeth. More specifically, this invention relates to a passive dental post and core system having a flexible inelastic post, wherein the post is made from a material having a plurality of distributed fibers, such as, for example, medical grade optical fibers, other medical grade fibers or other fiberglass materials, which are held together in a matrix in a resin, such as a polyester resin or a vinyl ester resin.
In the preferred embodiment, the flexible post has a modulus of elasticity less than or equal to that of tooth dentin, to prevent widespread damage to a tooth in a traumatic event, when a conventional post would flex less than the tooth dentin, causing tooth fracturing where the flexible dentin violently contacts the inflexible conventional post.
Also in the preferred embodiment, the endodontic post of the present invention is cylindrical, rather than wedge shaped as in many non-metallic posts, because of its less stressful impact and its decreased wedging effect, which can cause immediate and/or residual root fractures.
While the fibers may be axially aligned, preferably at least one of the fibers extends non-axially aligned with respect to a straight axis extending from the apical end to the opposite coronal end of a root of a tooth.
For example, the fibers may be a bundle of fibers, a longitudinally twisted bundle, a twisted braid, a woven lattice, a helically wrapped bundle of fibers, or a composite of randomly dispersed fibers in a binder.
In this preferred embodiment, at least one of the fibers extends non-axially aligned with respect to the straight axis of a root of a tooth.
For example, in a bundle of fibers, while some of the fibers may extend parallel to the straight axis of the root, at least one or more of the fibers extend in an axial direction which is not parallel to the straight axis of a root of a tooth. That is, at least one or more of the fibers extends in a transverse or angled direction away from the straight axis of the root of a tooth.
With respect to a longitudinally twisted bundle, a twisted braid, a helically wrapped bundle of fibers, the twisting or helical wrap of the fibers causes many, but not necessarily all, of the fibers to extend non-axially. Concerning a woven lattice of fibers, while one set of fibers could extend axially parallel to the straight axis of the root, the other intersecting set of fibers extends in a direction which is non-axially aligned with respect to the straight axis of the root.
Even if most of the weft of a weave of a plurality of fibers extends parallel to the straight axis of the root, at least one or more fibers constituting the warp of the weave of fibers extends non-axially with respect to the straight axis of the root of the tooth.
Moreover, concerning a composite of randomly dispersed fibers, there is always the possibility of one or more of the fibers being axially aligned to the straight axis of the root of a tooth. However, in order to be randomly dispersed, at least one or more of the fibers extends non-axially with respect to the straight axis of the root of a tooth.
Preferably, the post is radio-opaque and bears a color simulating that of a natural tooth.
Rigid dental post and core systems are widely utilized to restore endodontically-treated teeth. Post and core restorations are routinely used to create an adequate foundation for the final restorative step, which may be a crown, inlay, or a fixed partial denture abutment. Generally, a post is provided for retention and lateral stability of the restoration. The core provides support for the crown. Two general types of post and core systems are known in the art: "active" or screw-in type systems and "passive" type systems. Active post and core systems mechanically engage the walls of the root canal and tooth dentin. Passive post and core systems are bonded in endodontically treated teeth utilizing cements and the like.
Two major problems are encountered when restoring an endodontically-treated tooth. Firstly, the tooth is more susceptible to fracture, and secondly, there is generally less coronal structure with which to work. The greater susceptibility of a tooth to fracture after endodontia may result from the tooth being more brittle. However, studies of the changing mechanical properties of pulpless teeth do not generally support this theory equating dryness with reduced mechanical strength. It appears that the greater susceptibility for fracture in an endodontically-treated tooth results from mechanical weakening of the tooth during root canal therapy and refinement of the root canal. Improvements in restoration techniques that reduce mechanical weakening are therefore desirous.
An endodontically-treated tooth is generally severely compromised either due to trauma or neglect. Thus, traumatic fractures, removal of old restorations and carious tissue, and preparation of root canal access may not leave enough tooth to maintain the "dome effect" of the tooth or to retain a crown.
The stress concentrations in a tooth resulting from the rigid post and core systems of the prior art also play a vital role in tooth fracture. Stress concentrations can be impacted through system design and/or restoration techniques. Various studies and investigations into the susceptibility of endodontically-treated teeth to fracture and the contribution of rigid dental post and core systems to such fracture have been conducted. "A Comparison of Intracanal Stresses in a Post Restored Tooth Utilizing the Finite Element Method", Cailleteau, Johnny G., Rieger, Monty R. and Akin, J. Ed, Journal of Endodontics, Vol. 18, No. 11, November 1992, pp. 540-544, reports that placement of a rigid post within a tooth alters the pattern of stress along the root canal as compared with an intact tooth. Instead of strengthening the tooth the post stiffens the coronal posted section and shifts the flexure point apically. The effect of this stiffening causes the nonposted apical portion of the tooth to deform at the post apex, resulting in a stress increase in that portion of the canal wall. Also, the cyclic loading and unloading of an incisor during mastication requires consideration of fatigue failure. Since the maximum bending stresses occur in connection with the apex of the post, any inclusions or defects within the wall of the dentin near the apical end of the post would create stress concentrations that increase the risk of a fatigue crack formation. Defects and microfractures introduced during endodontic treatment and post access preparation could become areas contributing to stress concentrations. Studies have also shown that more intact tooth structure provides better resistance to fracture than a metallic post. There is also evidence that stresses in the tooth tend to increase as the post diameter increases.
A flexible post eliminates these problems and a cylindrical flexible post performs even better. A post and core system utilizing a flexible post shifts the stress concentrations coronally, eliminates the introduction of defects during post access preparation and post placement, and leaves more of the tooth intact.
The main function of a post is to provide retention to the core. Relieved of its expectation to facilitate resistance to tooth fracture, the post can be designed to optimize its retentive properties. Several factors govern the retentiveness of endodontic posts. The shape of the post and its length are among the essential factors.
For example, unlike the preferably flexible cylindrical post of the present invention, tapered dowels have been found to be significantly less retentive than parallel-sided posts. While inflexible metallic posts are generally cylindrical and/or threaded, non-metallic resin-based posts are generally tapered, such as described in French Patent Publication No. 8,515,527 of Barbe et al, published Apr. 10, 1987 or U.S. Pat. No. 5,326,263 of Weissman, where a tapered cylinder is seated within a wide tapered resin base. Such tapering was believed to enhance removal of a first temporary post to be replaced by a permanent post. Weissman '263, also describes a temporary flexible post including a single fiber optic cable rod, which is removed from a reamed, wedged shaped drilled out tooth canal before installation of a permanent, inflexible post. The post of Weissman '263 also has the drawback of being smooth on its surface, to facilitate easy removal of the temporary post.
Weissman '263 also describes a flexible tapered post insertable within a converging, tapered, canal wherein the converging tapered canal is filled with a curable composite. It lacks any texturization of the surface, which helps to maintain a permanent post in position within a tooth canal.
U.S. Pat. No. 5,165,893 of Thompson discloses using a fiber optical plunger applicator to apply a liner adjacent to the inner surfaces of a root canal. It does not describe a permanent post as in the present invention.
A serrated 5.5-mm parallel-sided dowel was found more retentive than an 8-mm tapered post. Tapered posts, such as described in Barbe noted above, provide high shoulder stresses but have an undesirable wedging effect. The wedging effect results in part from the prior art placement of a straight rigid post in a naturally curved and varying diameter root canal.
Furthermore, active threaded posts are very retentive, but may impose too much stress on the tooth, especially compromised teeth.
Thus it appears that a flexible passive, textured, parallel-sided cylindrical post is a preferred structure for dental post and core systems. A flexible, passive, textured parallel-sided cylindrical post provides the previously-mentioned advantages in preventing tooth fracture and additionally permits the post to extend for a greater length into the root canal for improved retention.
In addition to post shape and length, adequate retention is a function of cementing mechanisms. Various cementing medium have been studied. Utilization of low viscosity resin cement in combination with smear layer removal can be considered a universal post cementation technique. In addition to good retention, this cementing technique offers the benefits of a cement with very little resistance to post insertion, thereby minimizing stresses applied to tooth structure during cementation. However, the invention of the present disclosure is not limited by the cementing process used.
Nevertheless, light sensitive cements, such as REVOLUTION.RTM., of E. N. D. Dental Products Company, Somerset, N.J., can only act when used with a translucent substance. Therefore, there is a need for a translucent endodontic post as well.
A major problem of dental posts for endodontic root canal therapy is the inelasticity of posts, even if partially flexible. For example, stainless steel posts have a GPa (giga Pascals) of approximately 190, and titanium posts have a GPa of approximately 100, wherein the higher the GPa number the less elasticity of the post. One attempt to solve this problem is a non-metallic, carbon fiber, unidirectional post known commercially as C-POST.RTM. of Bisco Company of Itasca, Ill,
However, its modulus of elasticity is approximately 21, as reported in product literature therefor, whereas natural tooth dentin has a lower modulus of elasticity of 18. Since the modulus of elasticity of the C-POST.RTM. exceeds the modulus of elasticity of natural tooth dentin in which the C-POST.RTM. is inserted, the C-POST.RTM. may cause a tooth to fracture because the C-POST.RTM. is less elastic than natural tooth dentin.
Therefore, there is an unsolved need for an endodontic post for root canal therapy wherein the post has a modulus of elasticity less than that of natural tooth dentin. As a result, such a post would have less a likelihood of fracture, and will reduce the need for subsequent re-doing of post and core therapy after a post fractures or extraction of any non-restorable teeth.
Other background art includes an elastic, wire pin having a plurality of flexible, radially extending fins along its length, as is disclosed in German Patent No. DE 3,643,219 to Weisskircher. While providing some advantages over the prior art rigid post, the "high degree of elasticity" of the Weisskircher pin will cause it to try and retain its initial shape in the root canal. During and after placement, flexing of the pin will cause the apical end of the pin to lay against the wall of the root canal. Stress concentrations in the tooth as known for rigid posts will thereby be induced. A pin formed from wire also has low retention characteristics and tends to rotate within the root canal. Radial fins are utilized in the Weisskircher disclosure to resist rotation of the wire pin. However, these radial fins may become further sources of stress concentrations and fatigue failure as the wire pin rotates. No prior art known to the present Applicants discloses or suggests a flexible post in a dental post and core system that is flexible and inelastic, i.e., that conforms to the shape of the root canal to eliminate the stress concentrations that facilitate tooth fracture.
Furthermore, U.S. Pat. No. 4,778,389 to Salvo discloses a dental post construction to eliminate lateral stress in a tooth wherein a rigid, split post is formed by parallel sections joined at a marginal top portion of the post head.
U.S. Pat. No. 5,073,112 to Weil discloses a dental post having an active portion and a passive portion. It also describes a combination sleeve and threaded post, wherein part of the post is threaded, and part is not threaded. A temporary light transmitting rod is inserted to provide light to a light activated composite cement.
U.S. Pat. No. 5,074,792 to Bernadat discloses a passive post and core system comprising a rigid peg disposed in a porous sheath formed of high-strength filaments, wherein the peg has a set of parallel radially extending fins extending from the peg. The filaments in Bernadat are found in the sheath surrounding the peg, not in the peg itself.
U.S. Pat. No. 732,922 of Clark describes a pin for teeth which is flexible, but only by virtue of the fact that the pin includes a base and two tapered pins extending from the base, with a space therebetween, so that the tapered pins can close toward each other within the space.
U.S. Pat. No. 4,952,150 of Schiwiora discloses a tooth root post which includes a tip of solid flexible metal or metal alloys. In contrast to Schiwiore '150, in the present invention, the root post is made of a plurality of metallic or non-metallic fibers, as opposed to a solid piece of metal.
U.S. Pat. No. 4,934,936 of Miller describes a serrated dental post. U.S. Pat. No. 622,670 of Dwight and U.S. Pat. No. 1,218,289 of Maker both disclose solid threaded posts with a core spacer neck extending therearound.
International Search Publication No. WO 91/07142 (PCT/FR90/00831) to Reynaud et al., which also issued as U.S. Pat. No. 5,328,372, discloses a dental post and core system having a post formed from equally-tensioned fibers of composite material. In Reynaud, the fibers of the composite material are all laid axially within the post and embedded within a resin. Because the fibers are equally tensioned and extend only axially aligned and continuous, any modification of the post in Reynaud may cause a major spreading, continuous, fault line crack in the resin of the post, thus losing integrity of the Reynaud post.
In contrast to Reynaud '372, in the present invention preferably at least one or more of the fibers extends in a direction which is non-axially aligned with respect to the straight axis extending from the apical end to the opposite coronal end of a root of a tooth. Because there is a plurality of directions with respect to the fibers, such as at least one fiber running non-axially, the possibility of a spreading, continuous fault line crack is significantly reduced, thereby achieving unexpected beneficial results not suggested in Reynaud '372. Also, while the Reynaud '372 post can be cut in length, it is contraindicated to shave or adjust the Reynaud '372 post in all directions so that the possibility exists of causing the carbon rods to develop axial fault crack lines.
Other background art includes U.S. Pat. No. 4,936,776 to Kwiatkowski, which discloses a translucent post and core structure to minimize gingival discoloration adjacent a dental restoration, and U.S. Pat. No. 3,949,476 to Kahn discloses a "direct" method of restoring an abraded or broken tooth.
Soviet Union Patent No. SU 1,457,914 of Feb. 15, 1989, to Stomatology Research Institute discloses a method of making a pin stump insert. Moreover, Soviet Union Patent no. SU 1,519,684 of Nov. 7, 1989 describes a threaded grooved tooth implant. Furthermore, Soviet Union Patent no. SU 1,277,950 of Dec. 23, 1986 discloses an electrochemical bonding procedure for coating dental pins.
West German Patent No. 1,541,209 to Kurer discloses the now conventional threaded, screw-in type active post.
U.S. Pat. No. 4,622,012 of Smoler describes a two part dental post system with an outer hollow sleeve post and an inner post insertable within the outer post.
U.S. Pat. No. 4,759,718 of Szeguary describes an active threaded post. U.S. Pat. No. 4,726,770 of Kurer, Swiss Patent no. CH669514 of Polydent, U.S. Pat. No. 4,696,646 of Maitland, and U.S. Pat. No. 4,631,030 of von Weissenfluh, all describe interproximal contact wedge tools for filling cavities in a tooth.
U.S. Pat. No. 5,088,927 of Lee describes a dental plastic member impregnated with metal to enhance x-ray pictures. U.S. Pat. No. 5,030,093 of Mitnick discloses a dental restoration apparatus including a material setting tool which includes an optical probe. U.S. Pat. No. 5,092,773 of Levy describes an apparatus for filling the apex of a root canal with a laser mettable material. U.S. Pat. No. 5,116,227 of Levy describes a laser operable canal forming tool.
French patent application no. FR 2,645,431 of Levy describes a laser tool for cleaning a root canal.
German Patent no. DE 3,411,366 of Neumeyer describes an optical probe for periodental treatment.
U.S. Pat. No. 4,684,555 of Neumeyer describes dental retention pins made of metal, plastic, porcelain or ceramics. However, Neumeyer '555 includes two layers, an inner layer and an outer coating layer. This is in contrast to the present invention, in which there is an even distribution of fibers through the endodontic post. As a result, the post of the present invention requires no outer bond assisting or enhancing layer, as is needed in Neumeyer '555. Furthermore, Neumeyer '555 is not concerned with providing a pin having a modulus of elasticity less than tooth dentin, as is the endodontic post of the present invention.
Other prior art includes U.S. Pat. No. 4,645,457 of Goldman which describes a method of cleaning a root canal prior to installation of a post therein and U.S. Pat. Nos. 4,990,090 and 5,145,373, both of Roane, which describe grooved and/or threaded endodontic posts.
U.S. Pat. No. 5,320,530 of Fong describes an endodontic apparatus for retrofill cavity preparation and U.S. Pat. No. 4,172,867 of Devault describes an index pin and die spacer combination for dental use.
Furthermore, U.S. Pat. No. 5,407,973 of Hasegawa describes a dental cold-polymerizing resin composition and U.S. Pat. No. 5,284,443 of Weil describes a method of inserting a removable light transmitting mandrel point temporarily within a deposit curable composite material, wherein the light transmitting member provides light to cure the material.
In addition, U.S. Pat. No. 5,007,837 of Werly describes a method of filling a cavity and U.S. Pat. No. 822,582 of Carmichael describes an attachment for natural teeth and method of forming the same. U.S. Pat. No. 4,778,388 of Yuda et al describes root canal posts.
European patent application publication no. 0076086 of Carse dated Apr. 6, 1983, describes a threaded dental pin having a threaded pin member and a synthetic resin having a sharing neck 18.
British Patent no. 1,302,022 of Technical Dental Developments dated Jan. 4, 1993 describes an improved dental crown which uses resin with metal particles for casting a crown. It is not for a permanent post.
French Patent publication no. 2,626,167 of Himmel assigned to Compodent Research and Applications Ltd., dated Jul. 28, 1989, also known as British Patent no. GB 2,214,087, describes a dental post pin and a method of making the pin. The dental post pin essentially includes a central filament of yarn which is axially aligned within a sheath of fiber containing resin. Himmel also describes a plastic, ceramic, carbon or glass central wick or filament surrounded by an outer sheath of resin which could have other fibers in it.
In contrast, in the present invention, the fiber bundles preferably are equally dispersed throughout the peg of deposit and are not limited to the central portion. Also, in the present invention, there is no differential of an outer sheath having denser fibers from the loosely packed fibers of the central core.
French Patent publication no. 2,587,197 of Reynaud dated Mar. 30, 1987, and U.S. Pat. No. 4,738,616, also of Reynaud, describe dental posts which are made up of a serial of conical parts that are joined together in a cylindrical conical fashion.
German patent no. DE 3,825,601 of Strobl dated Aug. 9, 1989, describes a dental reconstruction post for endodontics, wherein a fiber reinforced plastic is used. However, there is no mention of the need for imparting flexibility in the post. In Strobl, the fibers are used specifically to strengthen the post and increase rigidity, not to make the post more flexible, as in the post of the present invention.
For example, in paragraph 3 of the section of the patent application of Strobl entitled "State of Technology, with Sources", it is stated that the strength and rigidity of plastics can be increased significantly by incorporating high-strength fibers with a high modulus of elasticity.
In contrast, the endodontic post of the present invention has a low modulus of elasticity, and is thus flexible.
Furthermore, Strobl teaches a wedge shaped post, which increases wedging stress within the tooth. While Strobl discloses rigid, diagonally extending non-axially fibers in the crown stump attached a post, in the post itself the fibers are described as lying in the direction of the root pin, i.e. axially, unlike the preferred embodiment of the present invention.
French Patent publication no. 1,457,914 of Badische dated Dec. 8, 1965, describes a thermal plastic material.
Currently-marketed dental post and core systems such as the FLEXI-POST.RTM.; the DENTATUS POST.RTM., the RADIX POST.RTM. and the BRASSELEAR.RTM. screw posts all advocate screwing threaded rigid posts into straight paths machined into the tooth dentin. These present day posts are also generally formed from rigid metals such as steel, titanium and other alloys which do not flex in the same manner as a natural tooth. As noted before, this differential in flexibility between the natural tooth and the post may cause tooth fracture when the restored tooth is stressed during mastication or from trauma. These cast posts are subject to the same limitations and require an additional laboratory fee and an additional visit to the dentist to complete the procedure.
A means to quickly and easily identify the components of a post and core system is also needed in the prior art. Presently, there is either no color-coding of post and core systems or the color identification consists of an inconspicuous dot of color. Brightly-colored means of identifying post and core systems would significantly advance the art. The lack of a color protocol in the prior art creates confusion, eye strain and a sloppy work environment. The inability to readily identify each post and core by sight creates problems before, during and after the procedure is completed. Firstly, before the procedure is initiated the dentist and staff must select the post and core and isolate it from others that may be very close in size. During the procedure the dentist must carefully avoid confusing the selected post and core. After the procedure the used and unused devices must be readily identified for contamination control. Further, a post and core system installed by one dentist may later require an emergency or other procedure by a different dentist in a completely different part of the world. Color-coded identification would eliminate uncertainty and guesswork.
The post and core system of the present invention overcomes all of these limitations of the prior art.